The present invention relates to lamps and more particularly to lamps including a phosphor layer and a non-phosphor layer.
Various coatings of non-luminescent particulate materials have been found to be useful when applied as an undercoating for the phosphor layer in mercury vapor discharge lamps, including fluorescent lamps. The phosphor coating is disposed on the inner surface of the lamp glass envelope in receptive proximity to the ultraviolet radiation being generated by the mercury discharge.
Examples of non-luminescent particulate materials which have been used in fluorescent lamps such as, for example, aperture fluorescent reprographic lamps, include titanium dioxide, mixtures of titanium dioxide and up to 15 weight percent aluminum oxide, aluminum, and silver. Titanium dioxide is typically used in commercially available aperture fluorescent reprographic lamps.
In some instances a layer of a non-luminescent particulate material is used to permit reduction in the phosphor coating weight. See, for example, U.S. Pat. No. 4,079,288 to Maloney et al., issued on Mar. 14, 1978. U.S. Pat. No. 4,074,288 discloses employing a reflector layer comprising vapor-formed spherical alumina particles having an individual particle size range from about 400 to 5000 Angstroms in diameter in fluorescent lamps to enable reduction in phosphor coating weight with minor lumen loss. The lamp data set forth in the patent, however, shows an appreciable drop in lumen output at 100 hours.
U.S. Pat. No. 4,344,016 to Hoffman et al., issued on Aug. 10, 1982 discloses a low pressure mercury vapor discharge lamp having an SiO.sub.2 coating having a thickness of 0.05 to 0.7 mg/cm.sup.2. U.S. Pat. No. 4,344,016 expressly provides that the use of thicker coatings causes a reduction in the luminous efficacy due to the occurrence of an absorption of the visible light.
Other attempts to improve the performance of and/or to reduce the costs associated with the manufacture of mercury vapor discharge lamps have involved the use of more than one phosphor layer. While the inclusion of an additional phosphor layer may achieve the desired maintenance improvement or cost reduction, the use of an additional phosphor coating is typically accompanied by a decrease in Color Rendering Index (CRI) of the lamp including the additional layer of phosphor.